Mass spectrometers are often used in combination with a liquid chromatograph or a gas chromatograph. In the case of a mass spectrometer combined with a liquid chromatograph (LCMS), the mass spectrometer is used as the detector of the liquid chromatograph. In the liquid chromatograph, a sample liquid containing components to be detected is made to flow through a liquid chromatograph column, where the components are separated with respect to time while the sample passes through the column. The liquid (eluate) flowing out of the column is ionized by an interface, and the ionized component atoms or molecules are introduced into the mass spectrometer where the ions are separated by their mass to charge ratios. Ions of the components thus separated in the mass spectrometer are detected by an ion detector.
In a LCMS, the eluate is normally ionized under the atmospheric pressure, so that the interfaces are generally called an atmospheric pressure ionizing (API) interface. The electro-spray ionizing (ESI) interface and atmospheric pressure chemical ionizing (ACPI) interface are typical API interfaces.
In the mass spectrometers using API interfaces, the ionizing chamber is kept at substantially atmospheric pressure, but the mass spectrometer section, which includes a mass filter such as a quadrupole mass filter and the ion detector, must be kept at a very low pressure (or in a very high vacuum). Since the pressure difference is so large, several (usually two) intermediate vacuum chambers are provided between the ionizing chamber and the MS section, and the pressures of the intermediate vacuum chambers are gradually changed in order to keep the mass spectrometer section at very low pressure. The Unexamined Japanese Patent Publication No. 2000-149865 (which has matured to U.S. Pat. No. 3,379,485) describes one of such differential pumping systems.
In differential pumping systems, ions are passed from a chamber of a higher pressure to an adjoining chamber of a lower pressure through a small hole called an orifice. In order to keep the low pressure of the lower pressure chamber, it is preferable to prevent unnecessary gas molecules from passing through the orifice. But, on the other hand, the object component ions should pass through the orifice at as large an amount as possible in order to enhance the detecting sensitivity. Conventionally, an ion lens applied with a DC voltage (an electrostatic lens) is provided before the orifice of an intermediate vacuum chamber, and the rear focal point of the ion lens is set at the orifice, so that object ions are converged and effectively injected into the orifice.
The conventional ion lens has the following drawback. In a mass spectrometer using an API interface, a considerable amount of gas (residual gas) remains in the ionizing chamber and subsequent low-pressure intermediate vacuum chambers, and it often happens that the sample ions contact the molecules of the residual gas. If an ion being converged by the ion lens contacts a residual gas molecule, the ion is deflected and is difficult to converge again to the orifice with the electric field by the ion lens alone. Since the concentration of the ions at the converging point (or near the rear focal point) of the ion lens is very large, the ions are highly probable to collide with the residual gas molecules, and to be prevented from entering the orifice and going to the subsequent chamber. This deteriorates the sensitivity of the sample analysis.